A known practice in vehicles is to arrange in a gearbox situated between the vehicle's engine and its powered wheels a device for operating the gearbox, which operating device comprises an operating rod and a shift finger firmly connected to the operating rod. The various gears in the gearbox are engaged and disengaged by moving the shift finger between various specified gear positions. The shift finger is usually moved in a conventional manner along an H-shaped grid where the crossbar of the H corresponds to a neutral position of the gearbox and the arms of the H correspond to the various specified gear positions. The H usually has four to six arms extending at right angles to its crossbar. The fact that the vehicle may be equipped with more than one gearbox connected in series makes it possible for more than four to six gear positions to be reached despite a gearbox having only four to six gear positions. The shift finger is moved along the H-shaped grid by a first cylinder adapted to moving the operating rod and the shift finger connected to the operating rod in the direction of the crossbar when the gearbox is in a neutral position, by the cylinder effecting movements of the shift finger via a pneumatic duct system. In other words, this cylinder controls the shift finger's lateral travel. When the shift finger is positioned in the arm corresponding to the specified gear which is to be engaged, the linear movement along the crossbar is halted and a second cylinder is adapted to rotating the operating rod about its axis, usually by approximately 30° from a central position at which the shift finger is oriented at right angles to the plane of the H, so that the shift finger moves transverse to the direction of the operating rod to a position on one of the arms whereby the desired gear is engaged by the shift finger pushing a shaft situated along said arm, i.e. this cylinder controls the shift finger's longitudinal travel. The gears in the gearbox are thus engaged and disengaged. The two cylinders are usually powered pneumatically by the vehicle's ordinary compression system.
A problem with existing operating devices is that the air pressure in the vehicle's ordinary compression system varies in different situations, with the result that the control of the shift finger is not effected at exactly the same pressure each time. Moreover, pneumatic systems are difficult to regulate, since the air is compressed when it enters the cylinder before the cylinder's piston moves, thereby making it difficult to control the shift finger. It is therefore necessary to provide systems for monitoring and comparing the shift finger's existing position with its expected position in the H-shaped grid.